1. Field of Invention
The present invention relates to the preparation of useful polymer compositions from mixtures of incompatible thermoplastic hydrocarbon polymers using a novel compatibilizing system. The invention also relates to novel compositions produced according to the process and articles fabricated from such compositions.
In general, thermoplastic hydrocarbon polymers are not readily combined into useful materials to produce new compositions since these polymers are of high molecular weight and contain large chain molecules.
In contrast to low molecular weight liguids, high molecular weight materials, such as hydrocarbon polymers, are only rarely "soluble" in each other. The reason is evident from basic thermodynamics. The fundamental driving force for solubility or miscibility is the change in free energy for the mixing process. This change must be negative for two materials to be soluble in each other. The free energy in turn has two components, an enthalpy or energy term and an entropy or statistical term. For small molecules, the entropy term usually dominates: the mixing of two components leads to a vast increase in possible distinguishable molecular arrangements, hence mixing of small molecules causes a large increase in entropy. The energy (enthalpy) change occurring as a result of mixing depends in turn on the net difference between the energy extended in separating the molecules of each component from each other versus the energy gained by the contact of the dissimilar-molecules of the mixture. If more energy is gained than extended, the mixing process is exothermic. If more energy must be expended than is gained, the mixing process is endothermic. Due to the large entropy gain in the case of small molecules, the components can have fairly large mixing endotherms and still be miscible, hence low molecular weight liquids can be fairly dissimilar and still remain homogeneous single phase systems when mixed.
This situation changes drastically as the molecular weight of both components increase: as the molecules become larger, the number of molecules decrease, fewer distinguishable arrangements can be made and the entropy effect rapidly becomes insignificant. Therefore, the energy (enthalpy) term dominates polymer melt blends. For this reason, unless two polymers have preferred (exothermic) molecular interactions, they will generally not be miscible.
Therefore, given the relatively high molecular weight of thermoplastic hydrocarbon polymers they are rarely soluble or even miscible in another. The vast majority of pairs of different thermoplastic hydrocarbon polymers form heterogenous blends when mixed and are therefore incompatible. The degree or extent of heterogeneity or incompatibility varies, of course, with the detailed structures of the given polymer pair. (For a more detailed discussion including descriptions of individual polymer systems, see "Polymer/Polymer Miscibility", by O. Olabisi et al., New York (Academic Press), 1979 and the literature references cited therein).
2. Description of Prior Art
Since most pairs of different hydrocarbon polymers are incompatible the most common approach which has been used to produce new hydrocarbon compositions is copolymerization, i.e., the polymer components are combined while still in a prepolymer state as monomers. This approach has created numerous commercially important materials such as random, block and graft copolymers from a wide range of monomers. However, copolymers are generally more expensive to make than the individual homopolymers and copolymerization does not provide a solution to the general problem of making useful blends from already existing polymers.
Another method of producing new hydrocarbon compositions was developed in the 1950s by W. F. Watson and coworkers at the British Rubber Producers' Research Association, as noted for example in British patent No. 828,895 dated May 7, 1956 and the article entitled "Polymerization of Admixed Monomers by the Cold Mastication of Rubber", Transactions I.R.I., vol. 34(1), 1958, pp. 8-19. These researchers demonstrated that the addition of monomers capable of free radical addition polymerization to a rubber during intensive mixing conditions can lead to formation of graft copolymers with unique and useful properties. Initiation of the grafting reaction is caused by free radicals formed by mechanical and thermal rupture of the rubber molecules during the intensive, high shear compounding and at least part of the monomer is polymerized into branches off the original polymer. However, this method does not produce new compositions from combinations of existing high polymers.
However, the same researchers demonstrated that it is possible under controlled conditions of high intensity compounding to create grafted interpolymers of two different rubbers. See, British patent No. 832,193 dated May 7, 1956 and the article entitled "Production of Natural Rubber Synthetic Rubber Interpolymers by Cold Mastication", Transactions I.R.I., vol. 33, 1957, pp. 22-32. It is today common practice in the rubber industry to use formulations containing more than one rubber type. It is important to note, however, that in addition to the interlinking which occurs during the intensive compounding conditions employed, rubber compounds must further be cured in order to achieve useful mechanical properties. The curing process creates additional polymer/polymer interlinking. Thus, blends of thermosetting polymers such as sulfur-cured (vulcanized) rubber compositions produced from blends of different rubber elastomers have been in commercial use for many years.
In contrast, attempts over the years to create useful blends of thermoplastic polymers by high intensity mixing have been less successful, even for polymers as close in composition as the olefin homo and copolymers. Illustrative of this point, is the article by R. E. Robertson and D. R. Paul entitled "Stress-Strain Behavior of Polyolefin Blends", J. Applied Polymer Science, Vol. 17, pp. 2579-2595, 1973. FIGS. 1, 3, 4 and 5 in this article show a precipitous drop in ductility, as measured by the elongation at break, for blends of polypropylene homopolymer with either low density polyethylene (LDPE) or high density polyethylene homopolymer (HDPE) in comparison with the properties of the components. Even blends of LDPE and HDPE suffer a noticeable loss in ductility on blending as compared to what would be expected from the mean values of the neat constituents. For polymers of more diverse composition, the problems are generally much worse. See, the article by D. R. Paul et al. entitled "The Potential for Reuse of Plastics Recovered from Solid Wastes", Polymer Eng. and Science, Volume 12, pages 157-166, 1972, covering the properties of polyethylene (PE), polystyrene (PS) and polyvinylchloride (PVC) blends. The data in this article (see, for example FIG. 4), clearly show that not only the ductility, but also the strength of the compositions is severely impaired by blending.
Another approach to the problem of blending preexisting hydrocarbon polymers has been to add free radical initiators during compounding. For example, U.S. Pat. No. 3,261,885, dated July 19, 1966, describes the preparation of blends of polyolefins and polyamides by hot compounding in the presence of a peroxide as a free radical initiator. Although these techniques often can provide compositions which have better mechanical properties than simple blends compounded together without the use of free radical initiators, these processes run to completion rapidly and are very difficult to control. The production of a consistent product grade is therefore, difficult. The products may also be partially crosslinked which makes them difficult to fabricate by injection molding and extrusion. Another disadvantage of the use of free radical initiators is that the decomposition products of the initiators are odoriferous materials which produce an unpleasant smell in the product.
An additional approach has been to add a "compatibilizing" component such as a lubricant or soap or, preferably, a copolymer containing chain segments similar or identical to one or both of those in the component thermoplastic polymers Examples of low molecular weight compatibilizing additives are given in U.S. Pat. No. 4,251,424, where sulphonamides and low molecular weight olefin/acid adducts are used to help compatibilize polyamide/modified polyethylene blends. In U.S. Pat. No. 4,283,459, aliphatic alcohols are used to help compatibilize PE/PP blends. Further examples of pol-ymeric compatibilizers are given in Japanese patent No. J58007440-A and in Japanese patent No. J60020947 A, where gummy, random ethylene/propylene copolymers or polyolefin-modified liquid- rubbers, respectively, are used to help overcome the inherent incompatibility of polyethylene/polypropylene (PE/PP) blends.
The principal disadvantage of this technique is that it is rather ineffective. Usually such large amounts of compatibilizing bridge additives or copolymers must be used that the properties of the additive severely affect the property profile of the blend.
Still another approach has been to introduce functional groups into the component thermoplastic polymers by pre grafting or by copolymerization. By selecting complementary substituents for the polymer pair to be compatibilized so that the substituents have a strong interaction, up to and including the formation of chemical bonds, it is possible to obtain polymer blends with significantly improved mechanical properties. An example of this approach is given in U.S. Pat. No. 3,299,176, dated Jan. 17, 1967. In this case, two polyolefins are made compatible by introducing acidic groups in one of the olefin polymers and basic groups in the other polyolefin. A more recent example is described in International Application PCT/US84/02122 entitled "Polymer Blends Containing a Polymer Having Pendant Oxazoline Groups", filed Dec. 26, 1984. This patent application describes the use of pendant cyclic imino-ether groups in one polymer and the use of complementary functional groups (such as carboxyl, hydroxyl or amine, capable of reacting with the imino ether groups) in another polymer. In this case, a covalent bond is formed during compounding connecting the two component polymers and compatible blends with significantly improved properties can be made.
The principal disadvantage of the functional substituent approach is that it is costly. Since the component thermoplastic polymers must be modified by pre-grafting or even by copolymerization, these extra process steps can add substantially to the cost of the final blend. Further, one wishing to practice this technique must inventory specialty polymers solely for the purpose of preparing thermoplastic blends, since the modified polymers often would not be cost effective when used by themselves.
The general problem of poor mechanical properties in blends of preexisting hydrocarbon polymers is further aggravated when the blends contain fillers, which often lead to a sacrifice in important mechanical properties such as ductility and impact strength.